Mutation, point: A single nucleotide base change in the DNA. A point mutation may consist of the loss of a nucleotide, the insertion of an additional nucleotide, or the substitution of one nucleotide for another.
The first point mutation discovered in humans involved the substitution of one nucleotide for another. It was in sickle hemoglobin, the molecular basis for sickle cell trait and sickle cell anemia. The mutation results in an amino acid change from glutamic acid to valine, from normal adult hemoglobin (hemoglobin A) to sickle hemoglobin (hemoglobin S).